This invention relates generally to d.c.-to-d.c. converters which convert one direct-current voltage into another, and particularly to a d.c.-to-d.c. converter for providing a constant output voltage through feedback control. More particularly, the invention concerns a d.c.-to-d.c. converter of the type capable of operation in either of two different modes (e.g. different numbers of switchings per unit length of time) according to whether the converter is loaded normally or less than so.
The d.c.-to-d.c. converter of the kind under consideration comprises a transformer having a primary winding connected across a d.c. power supply via an on-off switch, a rectifying and smoothing circuit connected to the secondary winding of the transformer for providing a unidirectional output voltage, an output voltage detector circuit connected to the rectifying and smoothing circuit, another detector circuit for detecting the magnitude of the current flowing through the switch, and a switch control circuit for on-off control of the switch according to the outputs from the output voltage detector circuit and the switch current detector circuit.
There are two familiar switching methods. One, known in the art as the ringing choke converter method, is such that the switching frequency is made higher with a drop in the power requirement of the load. Another involves pulse duration modulation; that is, the switching pulses remain unchanged in repetition frequency but are reduced in duration with less power consumption by the load. The ringing choke converter method is objectionable for the very high switching frequencies that unavoidably occur during converter operation, particularly under light load. Such high switching frequencies have made inconveniently high the ratio of the switching loss, or loss of power due to switching, to the power actually consumed by the load.
According to switching control by pulse duration modulation, on the other hand, the usual practice is to set the switching frequency as high as, say, 100 kHz with a view to the reduction of power loss at the transformer under normal load, as well as to the size reduction of the converter. This high switching frequency is maintained when the converter is operating in light load mode, too, making the number of switchings per unit length of time unnecessary high for that mode.
Japanese Unexamined Patent Publication No. 2000-23458 represents a solution to this poor efficiency of the prior art d.c.-to-d.c. converters under light load. The solution is such that the converter is operated at intervals under light load. Switching loss is lessened through overall reduction of switchings during the light load operation, although, admittedly, stability in output voltage is somewhat sacrificed. Another solution is found in Japanese Unexamined Patent Publication No. 9-140128, which teaches use of a lower switching frequency under light load than that under normal load.
Improvement in converter efficiency by switching between either set of two different operating modes depends upon constant monitoring of the variable power requirement of the load and, above all, accurate ascertainment of a level at which the operating modes are to be switched from one to the other. Although some suggestions have been made to this end, they are mostly unsatisfactory in either the simplicity of construction or the reliability of operation.
The present invention seeks, in a d.c.-to-d.c. converter of the kind defined, to accurately detect the power requirement of the load thereon by simpler circuit means than heretofore, in order to make switching control accordingly.
Stated in brief, the present invention concerns a d.c.-to-d.c. converter of the general construction comprising a switch connected between a pair of converter input terminals via inductance means such as a transformer, a rectifying and smoothing circuit connected to the inductance means for providing a d.c. output voltage to be applied to a load, and an output voltage detector circuit for detecting the output voltage.
More specifically, the invention provides, in the d.c.-to-d.c. converter of the general construction set forth above, a combination comprising a switch control circuit connected between the output voltage detector circuit and the switch for delivering to the latter the series of switching pulses of durations controlled according to the converter output voltage in order to keep the converter output voltage constant. A flyback period determination circuit is connected to the inductance means for providing a flyback period signal indicative of a flyback period during which a flyback voltage develops across the inductance means after the switch has been turned off each time. The flyback period determination circuit has an output connected to a load magnitude discriminator circuit, to whose another input is connected a reference period generator circuit which provides at least one prescribed reference period of time for comparison with the successive flyback periods. Inputting the flyback period signal and the reference period signal, the load magnitude discriminator circuit determines that the converter is loaded normally if each flyback period is longer than the reference period, and lightly if otherwise.
The output from the load magnitude discriminator circuit, indicative of normal or light loading on the converter, is fed into the switch control circuit. This switch control circuit is equipped to make on-off control of the switch in either of a selected set of two different modes depending upon whether the converter is under normal or light load. Several sets of two different switching modes are possible according to the invention. For example, in one set of such modes disclosed herein, the switch is driven at one repetition frequency when the converter is under normal load, and at another, less frequency when it is under light load. In another set the switch is driven at a frequency in inverse proportion to the load magnitude when the converter is under normal load, and at a fixed frequency less than the minimum of the normal load frequencies, when it is under light load. In still another set the switch is driven at a fixed frequency when the converter is under normal load, and at intervals at that frequency when it is under light load. In yet another set the switch is driven at a frequency in inverse proportion to the load magnitude when the converter is under normal load, and at intervals and at a fixed frequency less than the minimum of the normal load frequencies, when it is under light load. All such sets of modes are alike in that the average number of switchings per unit length of time is less under light, than under normal, loading.
Thus the invention advocates determination of whether the converter is under normal or light load from the duration of the flyback voltage. The load magnitude is accurately ascertainable in this manner by means comprising the flyback period determination circuit, the reference period generator circuit and the load magnitude discriminator circuit, which are all simple in construction and easy of fabrication with familiar electronic devices only.
According to a further feature of the invention, the reference period generator circuit provides two different reference periods for comparison with each flyback period by the load magnitude discriminator circuit. The switch is driven in light load mode when each flyback period becomes less than the first reference period. Once the light load mode is set up, that mode is maintained as long as the flyback period does not become longer than the second reference period which is longer than the first. Moreover, once the normal load mode is reestablished, this mode is maintained as long as the flyback period does not become less than the first reference period. In short the load magnitude is determined hysteretically, affording smooth transition between the two modes even in the event of a gradual change in load magnitude.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.